The in-plane switching (IPS) of a liquid crystal display (LCD) as depicted in FIG. 1 was invented more than 3 decades ago. Unlike the twisted-nematic (TN) cell where the applied electric field is in the longitudinal (length-wise) direction, the IPS electric field is in the transverse (cross-wise) direction. The liquid crystal (LC) molecules are rotated in the same plane. As a result, the IPS mode exhibits a wide viewing angle and high contrast ratio, and has been used widely for desktop computer monitors and LCD television monitors.
Referring to FIG. 1, the polarizer 10 axis is parallel to the LC alignment in the bottom plate 11 and the analyzer 12 is crossed. The LC alignment, or rubbing direction 13 is at an angle Φ to the electrode 14, as shown.
Soref published the first IPS concept in Applied Physics Letters, Vol. 22, p. 165 (1973) and Journal of Applied Physics Vol. 45, p. 5466 (1974). In 1992, Kiefer et al extended such operating principle to display devices; results were published in Japan Displays, p.547 (1992). In 1995, Oh-e and Kondo applied such IPS mode to the thin-film-transistor liquid crystal display (TFT-LCD). Two papers have been published by M. Oh-e and K. Kondo in Applied Physics Letters, Vol. 67, 3895 (1995) and M. Oh-e, M. Yoneya and K. Kondo, Journal of Applied Physics, Vol. 82, 528 (1997). Since then, many papers have been published on this topic.
In the IPS mode, two types of LC alignments are considered: the homogeneous cell as shown in FIG. 2a and the π-cell as shown in FIG. 2b. In FIG. 2a, homogeneous cell 20, shows the LC pre-tilt angle (α) on each surface 21 is tilted to the same direction so that the bulk LC directors are uniformly tilted to a direction. On the other hand, the π-cell 22 in FIG. 2b has opposite pre-tilt angles at the boundaries of top substrate 23 and bottom substrate 24. All the abovementioned IPS modes use a homogeneous LC cell arrangement. The π-cell 22 is symmetric in the left (L) and right (R) viewing directions, but the homogeneous cell 20 is asymmetric.
Bos and Koehler/Beran first published the π-cell configuration in Molecular Crystals and Liquid Crystals, Vol.113, p.329 (1984). The π-cell eliminates the backflow of the LC directors and achieves a fast response time. However, their applied electric field is in the longitudinal direction. As a result, a bias voltage is needed to overcome the domain problem and phase compensation films are needed to achieve a wide viewing angle.
In U.S. Pat. No. 6,356,329 B1, Tamatani et al. used the π-cell configuration in the IPS electrodes. The viewing angle is as wide as an IPS homogeneous cell. An extra benefit of the IPS-π cell over the IPS-homogeneous cell is that its color shift at large oblique angles is reduced because of the symmetry of molecular alignment in the horizontal direction. However, in the U.S. Pat. No. '329 issued to Tamatani et al, the LC orientation angle is set at 10° with respect to the electrodes. The advantage is that the on-state voltage is low. However, the response time is slow. Response time is a critical issue to computer and television applications.
S. H. Lee et al. published a paper in Soc. Inf. Displays, Vol. 28, 711 (1997) reporting on the rubbing angle dependent response time. The LC mode studied by S. H. Lee et al. was the homogeneous cell, not the π-cell used in the present invention. Moreover, results of the research by S. H. Lee et al. indicate that after Φ˜20°, the response time basically saturates. Thus, the response time reaches a certain minimum value and does not improve anymore even with further increases of Φ. On the contrary, in the present invention, it was unexpectedly discovered that by increasing the LC orientation angle (i.e., rubbing angle) to 30–40°, the response time is shortened by 2–3 times as compared to that at Φ=10°; the brightness is improved more than 33%, and a more linear gray scale is obtained, while preserving a wide viewing angle.